The present invention bears a relation to a charged particle microprobe system and, in a preferred embodiment illustrated, is included in a field emission electron column wherein the beam of charged particles (electrons) is caused to scan the surface of an object according to a predetermined pattern. In the usual embodiments of such field emission electron systems, the electron beam is caused to initially scan a specimen in a raster pattern somewhat similar to that utilized in commercial television monitors, that is, the electron beam is caused to scan the object to be irradiated much similar to the electron beam scanning the face of the cathode ray tube. The field emission electron optical column has gained wide usage in the embodiments as scanning electron microscopes which may be seen by reference to U.S. Pat. Nos. 3,678,333; 3,842,272; 3,931,517; 3,931,519; 3,925,664 and others, commonly owned by the assignee of the present invention. As may be seen in one or more of the aforementioned patents, the charged particle beam may be laid down in an interlaced pattern either in a sequential fashion or in a non-sequential fashion with interlace ratios anywhere from 1:1 one-to-one to any order of ratio such as 8:1 to 16:1.
In a present embodiment of the invention, subsequently described, the field emission electron optical column may be utilized in multiple fashions including utilizations as an electron microscope as previously referred as well as a beam-writing instrument which is capable of irradiating a specimen in such a fashion as to actually "write" thereon. In a beam-writing configuration, the actual bombardment of the beam upon the specimen causes some change in the physical characteristics of the specimen such that the changed character of the beam thereon can be later "read". In the embodiment of the invention subsequently described, the beam is caused to scan the specimen to be irradiated in the usual raster pattern such that were the beam not interrupted from bombarding the object, the entire raster pattern would be painted or written upon the surface of the specimen. In the disclosed invention, however, a deflection mechanism is included in the electron columns that for every portion of the normal scan where it is desired that the beam of charged particles not reach the object, it is deflected from its normal path and then where the beam "writing" is again desired, the deflection circuitry is de-activated and the beam returns to its normal scan. By such mechanism, alpha numeric characters might be written on microfilm, or in another application, a circuit may be etched on a suitable substrate or resist.
As may be recognized, an extremely fast deflecting mechanism is required here since the scanning of the corpuscular particles is already at television rates and are capable of laying down a full raster on the object in approximately 1/60 of a second.
Thus, the essence of the present invention lies in the circuit for forming essentially the rectangular-wave type pulse wherein the circuit exhibits a capacity for a very rapid rise time from a steady state level to a high steady state DC voltage and then a very rapid fall time again back to the first steady state or "on" condition. One of the essential characters of the circuit in order to accomplish these relatively instantaneous responses is its high resistive character. The resistive character is further accented in the usage of a switching device which in and of itself has an essentially resistive physical characteristic. This resistive maximization results from putting the switch in series with a large current-limiting resistive load. Further, the voltage drop across the switching device is minimized such that its change of state from "off" to "on" and back to "off" (or vice versa) provides little reflective influence on the circuit proper.
Further, by using a high current-limiting value and a resistive switch exhibiting a little voltage drop during its "on" condition, current flow in the circuit is minimized such that the inherent current heating of the components and thermal impact thereof is minimized, providing again, an additional degree of stability in the circuitry not otherwise achieved.
In the present embodiment of the circuit which is utilized in either an electron microscope or an electron microprobe wherein the microprobe is used in a beam-writing function, high capacitance may be experienced in the associated circuitry which the switch circuit is required to drive. In the case of the microprobe devices, deflection plates are required between which a potential is established in order to deflect the electron beam off its normal axis. It should be realized that when this voltage is applied to deflect the beam, some period of time is required for a "charge-up" of the deflection plates and thus slowing the speed of any switching function. Where high speeds are required, naturally, one must minimize this "charge-up" delay and maximize the speed with which the circuit operates.
Thus, in the illustrated embodiment, an additional function is included to compensate for this characteristic. To provide this additional compensation, a second switch is included which is connected between the deflection plates potential source, and the plates, essentially shunting out the current-limiting resistor momentarily. This switch is actuated with the "off" actuation of the first switch. However, it is maintained "on" only for a transient period of time which is determined by an RC differentiator circuit. The time constant of the RC circuit is such that the direct application of the deflection plate, the potential from the source directly to the plates occurs only during this otherwise slow charge-up period and, in effect, provides a very low impedance source for the deflection plates during this period of time; thus, squaring off as it were the rise time and rise trace of the voltage applied to the deflection plates thereby speeding the response time. As one skilled in the art reads the following description of one of the preferred embodiments, additional areas of application for the quick response circuit will become evident.